Simultaneous phase-shifting interferometers solve many problems associated with full-field interferometric measurements such as surface profiles of optical components. They are capable of acquiring all required information at the same instant—and diminishing influence of environmental vibrations, air thermals, and moving test surfaces.
In particular, some of these instruments utilize polarization as a means for obtaining required phase shifted images. Typically this involves illuminating the object with a linearly polarized light beam while the reference surface such as reference flat or sphere is illuminated with orthogonally polarized light. After recombining these beams, additional polarization components allow simultaneous observation of phase-shifted fringe patterns used to recover the test object's shape. Examples of these interferometers are shown in our earlier U.S. Pat. No. 8,004,687.
However, these types of interferometers are non-common path either geometrically and/or polarization wise. This means the test and reference wavefronts are affected in different ways as they travel through the optics of the interferometer system. These effects can result in measurement errors.
The information contained in these two beams is carried as a difference in phase of the optical waves which is influenced by polarization dependent properties of optical components in the interferometer. Test and reference beams can be subject to different influences as they propagate through the system and they contribute to the measurement error. Examples of such influences are birefringent materials, coating properties, optical stress, angle of incidence of the polarized light with respect to the object surface or object ti surface properties.
Typically, these errors are removed by measurement and subtraction of a known calibrated reference optic. Unfortunately, they can be pricey and their error characteristic must be superior to that of the actual test object. Removing these errors typically requires the use of expensive precision optics and additional test procedures.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.